The field of the invention relates to the field of plastic gears and plastic gear boxes for gears, and, in particular, to the formation of plastic helical gears and gear boxes utilizing those helical gears for toys and other small motorized devices.
There is a continuing and long-felt need for inexpensive, plastic gears for use in toys and similar applications. Gears transmit rotational movement and torque forces. Gears may be used to convert the high-speed, low torque output of a rotating electric motor to a low-speed, high torque output of a wheel drive shaft for a toy car. They also may be used to move the mechanical arms of, for example, a toy construction crane. Gears for toys should be safe, inexpensive and wear resistant. Plastic gears are suitable for toys because they are safe as they do not have sharp edges (as do metal gears); may be inexpensively formed by injection molding processes, and are tolerant of the dirt and wear encounter by toys, especially toy cars, trucks and construction vehicles.
There is also a long-felt need for an inexpensive helical gear formed by plastic injection molding. Helical gears can be used in conjunction with a worm gear to transmit rotation and torque from a rotating worm gear to a helical gear. Helical gears have a variety of applications, including engaging a worm gear mounted on the shaft of a small electrical motor to turn the gears of a gear box.
By using helical gears to engage a worm gear on a motor shaft, a motor is not constrained to be mounted perpendicular to the plane of rotation of the gears in the gear box. A motor with a spur gear must be mounted so that its output shaft is perpendicular to the plane of rotation of the gear. This constraint on the mounting of a motor having a spur gear may cause difficulties in arranging the motor and gear box in a small space, such as within a toy vehicle. A motor with a helical gear may be mounted parallel to the axes of rotation of the gears in the gear box. Having the flexibility to orient the motor in relation to the gears is particularly advantageous in a small toy vehicle where the spaces for mounting a motor are limited.
In addition, a helical gear may be used to reduce the rotational speed of the motor shaft to a lower speed of a wheel rotation, with fewer gears than would be practical without helical gears. Reducing the number of gears allows gear boxes to be more compact and have fewer components, than do prior gear boxes with many spur gears. With a standard pair of spur gears their relative speeds of rotation depend on the ratio of the number of gear teeth on each gear. The number of gear teeth on a spur gear depends on the diameter of a gear. A helical gear may be rotated by (or may rotate) a worm gear, which has a small diameter relative to a spur gear. The pitch (or angle of the gear teeth on the helical and worm gears relative to the screw axis) determines the speed of rotation of the helical gear being driven by the worm gear. A relatively-small screw and helical gear assembly may be used to dramatically reduce the rotational speed of a motor down to a speed suitable for the wheels of a toy car. By using a relatively-small pitch angle, e.g., 6 degrees, on the helical and worm gears, the rotational speed reduction from the rotating speed of the motor gear to that of the driven helical gear, may be much greater than could be practically accomplished with a pair of spur gears.
Helical gears have been difficult to form by plastic injection molding. To form a gear by injection molding, a gear cavity must be formed in the mold. Liquid plastic is rapidly injected into the mold cavity and the plastic is allowed to solidify during a cure period. Once the plastic has hardened, the mold is split apart and the plastic gear removed. This process of injecting liquid plastic curing, opening the mold and ejecting a gear is repeated rapidly in a typical commercial injection mold apparatus. Difficulties arise during the molding process such as: the plastic flows into surface imperfections of the mold cavity; the metal that forms the mold cavity may corrode; the volume of plastic injected in the mold cavity may be excessive; the cooling period needed to hardened the injected plastic may be inadequate; and the ejection process may be too fast for helical gears.
These and other problems with the plastic injection process have in the past made it difficult to form helical gears at sufficiently fast production rates. The production rates must be fast to satisfy the demand for plastic gears and to reduce the cost of manufacturing these gears. If the production speed is too slow, then the cost to manufacturer plastic gears, especially helical gears, becomes greater than the cost to use metal gears or other alternatives to plastic gears. If the production of plastic gears is prone to malformed gears or gears that do not properly eject from the mold, then the cost to make the gears becomes excessive. In the past, helical gears have not been made from plastic because the production rate has been inadequate to meet the demand for gears used in toys and the cost has been greater than the cost of metal gears or of other alternatives to plastic gears. Accordingly, there has been a long-felt demand for plastic helical gears.
In the present invention, a helical gear is formed by an injection molding process in which the mold cavity is formed of mirror finished hardened stainless steel. The mirror finish prevents the plastic of the gear from sticking to the mold cavity, and the stainless steel is corrosion resistant. The injection of plastic is carefully metered to dose the proper amount of plastic and to apply the proper pressure to the plastic. By properly metering the plastic injection the invention avoids the problems associated with over-packing the mold cavity with plastic, such as gear warpage and excessive internal stresses in the gears.
Once the plastic is injected, the cooling period allotted to a helical gear is longer than the cooling period for straight gears. Moreover, the pin is balanced and straight such that the gear slides smoothly off the pin as the gear ejects from the mold. A sleeve that forms a collar to the ejection pin slides along the pin to eject the gear from the mold. The ejection of a helical gear is conducted at a slower speed than the ejection of the straight gears. Helical gears have gear teeth that are at an angle with respect to the gear axis. The ejection of gears from a mold is in the direction of the axis of the gear. For a straight gear, the ejection is a straight, non-rotating movement in the direction of the gear axis. To eject a helical gear the gear must rotate slightly as the gear moves out of the mold, to accommodate the angled gear teeth. To allow the helical gear to rotate as it is ejected, the gear must be more slowly ejected from the mold than the ejection speed used for straight gears. If the helical gear is ejected too quickly, the gear teeth may be damaged or stripped off. By slightly reducing the ejection speed of the gears and implementing the other features of the invention, helical gears can formed by plastic injection molding at production rates sufficient to produce low-cost gears for toys and other mass-produced products.
There is also a long-felt need for gear boxes that may be conveniently arranged in or integrated with toy vehicles and other small devices. A gear box transmits rotation and torque through an assembly of intermeshing rotating gears. An input shaft to the gear box transmits a drive rotation to the gears and to an output shaft(s) from the box. As the drive rotation causes the intermeshing gears in the box to rotate, the rotational speed of each of the gears will vary depending on the gear teeth ratios of each pair of gears. The torque and rotational speed of the output shaft will be in proportion to the input shaft speed and torque, where the proportional relationship depends on the arrangement of gears between the input and output shafts.
An embodiment of the present invention is gear boxes that entirely encase the gears, so that dirt and dust cannot easily come between the gears. The gear boxes may also be integral with the housing of the toy to minimize the components in the toy and to reduce manufacturing costs. If an integral gear box is not practical, then an encasing gear box may be designed to fit easily in the housing of the toy adjacent to the wheels, mechanical arm or other component to be turned by the gear box. Accordingly, the gear boxes of the present invention seal the gears against dirt and dust, and may be integrated into the plastic housing of a toy or for a separate housing mounted within the toy.
Furthermore, the axles used for gears have been short metal shafts that slide through an axle bearing in the gear and are supported by a pair of axle support posts on either side of the gear. The plastic support posts used in toys tend to flex to allow the metal gear shaft to snap into place in the gear box. These gear shafts have a tendency to pop out of their support posts after the toy has been in play. Another technique for mounting an axle shaft is to slide the shaft through an aperture in one or both of the support posts. This technique for mounting an axle shaft suffers from the problem that the axle may slowly slide out through the aperture in the support post as the toy is played with. Whether the axle pops or slides out of place, such movements of an axle in a gear box will cause the gears to become misaligned and render the gear box and toy inoperative. Accordingly, there has been a long felt need for a better axle for use in plastic gear boxes used in toys and other applications.
The present invention also includes an xe2x80x9cLxe2x80x9d axle, that may be incorporated into a gear box. The xe2x80x9cLxe2x80x9d axle is an advance over prior straight axle shafts used for plastic gears. The L-axle provides a straight shaft portion that extends through the axle bearing aligned with the centerline of the gear. The straight section of the L-axle may have a free standing end to receive the gear(s) during assembly of a gear box. The opposite end of the L-axle is bent at, for example, a right angle. The bent portion of the axle is seated in a slot recess in the wall of the gear box. A support groove in the slot recess tightly holds the bent portion of the axle in place. The slot recess and groove rigidly hold the axle such that the axle does not rotate or slide axially during use. Accordingly, the xe2x80x9cLxe2x80x9d axle solves the problems experience with prior straight shaft axles, which problems included axle rotation and sliding of the axle which caused the gears to fall out of alignment within the gear box.
The invention provides several advantages for gears, transmission assemblies, e.g., gear boxes, and gear linkages over the prior art including, but not limited to: improved safety, better resistance to dirt and grime, fewer components, especially metal components, lower manufacturing costs, and compact arrangements of motor and transmission gear assemblies. Safety is improved, especially for toys, because the invention reduces the number of small gears needed for a transmission assembly and thereby reduces the number of components that may be separated from a toy and inadvertently swallowed by child. Safety is also improved by having helical gears formed of plastic, which is less likely to cut a child, than would metal helical gears. The invention resists dirt and grime by encapsulating gears and motors in gear boxes. The lower manufacturing costs flow from forming helical gears from plastic, rather than metal, and reducing the number of gears needed by utilizing helical gears. In addition, compact arrangements of motors and transmission gear assemblies is achieved because the use of helical gears allows the motor to be arranged adjacent to the gear assembly.